Materials with highly-oriented atomic structures exhibit properties that are necessary for diverse applications, such as superconductors, optical devices, and microelectronics.
For example, superconducting ceramic films and coatings require highly-oriented atomic structures to maximize superconducting critical current density (JC). Current density is a vector quantity whose magnitude is the ratio of the magnitude of current flowing in a conductor to the cross-sectional area perpendicular to the current flow. Examples of such films and coatings include rare earth-alkaline earth metal-cuprates (i.e., YBCO), such as YBa2Cu3O7.
Optical devices, such as optic shutters and waveguide devices; and microelectronics utilize ferroelectric materials. A high degree of atomic orientation of ferroelectric materials is necessary in optical devices to minimize optical propagation loss. Such orientation is necessary in microelectronics to minimize dielectric constants, and to allow for stable and constant resistivity. Examples of ferroelectric materials include perovskite-type materials, such as, for example, Pb(Zr,Ti)O3 and (Ba,Sr)TiO3.
The superconducting ceramic and perovskite-type materials can be deposited as precursors onto supporting materials, termed substrates. Substrates can range from being a single crystal to being a whole article, i.e., tape, wire, wafer, etc. After the precursor is deposited, it is converted into a highly-oriented atomic structure. The particular substrate upon which a precursor is converted, and the particular conditions under which conversion takes place, significantly determine the type and degree of atomic orientation that a converted precursor, and thus its respective end-product, will exhibit.
In general, to achieve a high degree of orientation, a precursor is deposited on a substrate that allows for oriented crystalline growth, i.e., epitaxial growth, and that primarily produces end-products with no grain boundaries, or, in certain cases, low angle grain boundaries. Single crystalline structures, and in particular single crystals, allow for better epitaxial and heteroepitaxial growth and thus provide a higher degree of atomic orientation vis-à-vis polycrystalline structures.
For example, YBCO films, grown on epitaxial single crystals, have exihibited current densities of 4×106 amps/cm2 at 77K. In contrast, current densities are drastically reduced when YBCO films are grown on untextured polycrystalline structures. In such cases, the number of crystal grain boundaries are increased thereby producing “weakly linked” crystalline structures.
The inclusion of fluorine into precursors allows for the production of end-products with a high degree of atomic orientation. It is thought that fluorine enhances the transfer of the crystalline order of a substrate to the growing material, i.e., enhances epitaxial and heteroepitaxial growth. The incorporation of fluorine also allows for the use of more stable materials in the manufacture of the precursor. For example, barium metal is difficult to handle because it rapidly oxidizes in air, especially in the presence of water vapor. In contrast, YBCO precursors can be fabricated using BaF2, a material that is stable in air.
However, it has been found that the inclusion of fluorine in precursors can lead to the accumulation of hydrogen fluoride (HF) gas in reaction vessels during the conversion process. There are many obstacles associated with the accumulation of HF gas in reaction vessels.
For example, the accumulation of HF gas during the conversion of YBCO fluorinated precursors to crystalline YBCO is a rate-limiting step to such conversion (Physica C 353:14-22 (2001)). In particular, it is estimated that the HF equilibrium vapor pressure reaches about 10 milliTorr in a closed reactor. Such pressure is sufficient to stop the growth of an YBCO film.
Techniques to extract HF gas quickly enough to avoid the suppression of crystalline YBCO growth have been used. For instance, the flow of gas in a reactor has been increased to effect faster removal of HF gas by gaseous convection. However, for any economically meaningful length of conductor in a tubular reactor, the degree of gas flow required becomes unreasonably high.
The accumulation of HF gas during the conversion of YBCO fluorinated precursors to crystalline YBCO has other drawbacks. For instance, in order to achieve uniform and homogeneous growth of crystalline YBCO film, the HF gas should be uniformly extracted perpendicular to the surface of the growing YBCO film. This can be achieved for small samples with flat geometries by placing a suitably designed pump aperture over the film surface. However, for large areas, or for geometries other than flat, this method is not practical.
Additionally, the pump aperture extracts water vapor and oxygen that are required for the conversion of the precursor to crystalline YBCO. Thus, oxygen and water vapor must be re-injected. Since any component of gas flow parallel to the surface will cause the HF partial pressure to become non-uniform, the oxygen and water vapor must be re-injected perpendicular to the film surface. This can be approximately achieved hydrodynamically for laboratory-sized samples where the relevant distances are measured in mm. However, for large areas, or for geometries other than flat, the injection of the gases becomes impractical if not impossible.
Accordingly, there is a need in the art for an efficient method to remove HF gas from chemical fabrication processes that yields high quality crystalline end-products.